Let be a positive integer. Let be the sum of the natural divisors of (including and ). We say that an integer is superabundant (P.Erdos, ) if ,
Prove that there exists an infinity of superabundant numbers.
Solution
1. Define the function , where is the sum of the natural divisors of .
2. We need to show that there are infinitely many integers such that , . These integers are called superabundant numbers.
3. Consider the product , where denotes the -th prime number.
4. We claim that this product diverges. To see why, note that:
5. It is a well-known result that the sum of the reciprocals of the primes diverges. Specifically, the series diverges.
6. Since the product diverges, the function can get arbitrarily large.
7. To see why can get arbitrarily large, consider the values of at products of distinct primes:
8. Since the product diverges, can become arbitrarily large.
9. Because can become arbitrarily large, it must achieve a new maximum infinitely often. Each time achieves a new maximum, the corresponding is a superabundant number.